Interpretive Summary: In an effort to understand how feeding and bedding practices affect gaseous emissions from dairy farms, we conducted a study to quantify emission of ammonia and the greenhouse gasses nitrous oxide, methane and carbon dioxide from manure during a 77-day storage period. The manure was produced by dairy cows fed diets differing in forage to concentrate ratios but identical in crude protein concentration. Dietary treatments had no effect on emission rates. However, the presence of straw bedding in manure promoted the formation of a surface crust that became air-dried after about one month of storage and was associated with an altered pattern in gaseous emissions. In addition to its physical effects, the crust may have provided a growth environment for bacterial species that use ammonia and methane as substrate, reducing emission of these compounds, but promoting the production and emission of nitrous oxide and carbon dioxide. This information shows that straw bedding in dairy barns not only provides cow comfort but it also reduces gaseous losses during manure storage.

Technical Abstract:
Sixteen 200-L barrels were used to determine the effects of dietary forage to concentrate ratio (F:C) on rates of NH3-N, N2O, CH4 and CO2 emission from dairy manure during a 77-d storage period. Manure was obtained from a companion study where cows were assigned to total mixed rations that included the following F:C ratio: 47:53, 54:46, 61:39 and 68:32 (diet dry matter (DM) basis) and housed in air-flow controlled chambers constructed in a modified tie-stall barn. On day 0 of this study, deposited manure and bedding from each emission chamber was thoroughly mixed, diluted with water (1.9 to 1 manure to water ratio) and loaded in barrels. In addition, on days 0, 7, 14, 28, 35, 49, 56, 63, 70 and 77 of storage, rate of NH3-N, N2O, CH4 and CO2 emission from each barrel were measured. Data were analyzed as a randomized complete block with four replications. Dietary F:C ratio had no effect on manure DM, total N and total ammonia N (NH3-N + NH4+-N), and pH at the time of storage (mean ± SD 10.6±0.6%, 3.0±0.2%, 93.1± 18.1 mg/dL, and 7.8±0.5, respectively). No treatment differences were observed in overall rate of manure NH3-N, N2O, CH4 and CO2 emission (mean ± SD over the 77-d storage period; 117±25, 30±7, 299±62, and 15,396±753 mg/hr per m2, respectively). The presence of straw bedding in manure promoted the formation of a surface crust that became air-dried after about one month of storage and was associated with an altered pattern in NH3-N and N2O emissions in particular. Whereas NH3-N emission rate was highest on d 0 and gradually declined until reaching negligible levels on d 35, N2O emission rate was almost nil the first two weeks of storage, increased sharply to peak on d 35 and declined subsequently. Emission rate of CH4 and CO2 peaked simultaneously on d 7, but declined subsequently with a distinct pattern. In this study, C:N ratio of gaseous losses was 32:1 reflecting higher volatile C loss than volatile N loss during storage. Furthermore, the most important source of non-CO2 greenhouse gas emitted was CH4 until formation of an air-dried crust, but NO2 thereafter. Taken together, these results suggest that the formation of an air-dried crust resulting from the straw bedding present in the manure reduced drastically NH3-N and CH4 emission, but was conducive of N2O production and emission.